A study of changes on a molecular level which control cell properties, growth and differentiation, and are possibly involved in the transformation to malignancy as well. A cellular protein found bound to SV40 T antigen (TA) in cultures of SV40 transformed murine cells, and later in normal midgestation murine embryos, was thought, by analogy to TA, also to be involved in control reactions pertaining to DNA replication and cell growth and division. Since TA is known to bind to DNA, the behavior on calf thymus DNA-cellulose columns of extracts of cultured mouse cells was analyzed for information concerning the nature and properties of this 55,000 molecular weight (MW) protein (55K protein) in the presence of TA. Results were compared with those obtained from similarly treated extracts of embryonal carcinoma (EC) cells, which contain the 55K protein. Evaluation of the ability of the 55K protein (complex) to associate with DNA should provide a basis for understanding such interactions within the cell. It is hoped to gain a better understanding of the biological activity and function of such proteins and their contribution to the basic control mechanisms of the cell, as well as to develop better methods for their isolation and purification.